An example of the track configuration of the conventional optical disk will be described referring to FIG. 16. A plurality of groove tracks 11 and land tracks 12 are arranged alternately in a radial direction of a disk-like recording medium. Each track is wobbled by a small amount in the radial direction. Further, each track is divided into a plurality of circular arc sectors that are arranged in the radial direction, and in a leading part of each circular arc sector, a header 6 having address information used for identifying a recording area is placed. The headers 6 are arranged in the radial directions, i.e., are placed on radial lines. In this example, a width of each track is approximately 0.6 μm and a groove depth of each groove portion is approximately 60 nm. In this example, a length of the sector is approximately 6 mm, which corresponds to a user capacity of 2048 bytes. Each groove portion and each land portion are wobbled by amplitude of approximately 20 nm in the radial direction. The period of the wobble is set at 1/232 times of the sector length, namely, approximately 25 μm. This ratio 1:232 is chosen to correspond the period of the wobble to integral multiples of the length of the record data (channel bit length) such that a recording clock can easily be generated from the wobbles.
FIG. 16 shows details of a sector address header portion in a leading part of the track, namely, an identification information portion. In FIG. 16, pieces of the identification information are placed at two positions, a first position 631 and a second position 632, that are aligned in the radial direction, on the radial lines. The track connects to an immediately previous/subsequent track when making a round, i.e., the groove portion 11 to the groove portion 11 and the land portion 12 to the land portion 12. In this example, each identification information item corresponds to a recording area of an information track at its right. Further, identification information corresponding to the groove part information track 3 is placed at a first position 631, and the identification information corresponding to the inter-groove part information track 4 is placed at a second position 632. That is, pieces of the identification information are placed in such a way that the position thereof along the information track is different from those of adjacent tracks but agree with those of adjacent-but-one tracks. That is, when viewing on boundary lines between the land track and the groove track, the placement position of the identification information is divided into first and second areas, and the first and second identification information areas are used alternately one track by one track.
By this arrangement, for example, when the light spot 21 is scanned on the groove portion 11, the pits on the one boundary line are always reproduced to avoid the crosstalk among one track and the adjacent tracks. Therefore, it becomes possible that the address information placed on the prepits is reproduced without crosstalk. In this example, the address information of the prepits is recorded with an 8/16 modulation code (channel bit length=0.14 μm).
The identification information in the header part is composed of small dimples (pits), which are formed due to the unevenness of a substrate or the like together with the grooves etc. when the disk is fabricated.
A phase change type recording film (GeSbTe) is used as a recording film, and a record mark is shaped as an amorphous region.
The foregoing conventional example is described in detail, for example, in Japanese Patent No. 2856390, etc.
However, in applying the above-mentioned conventional technique to high-density recording where recording/reproduction is performed with a blue light source, it was difficult to form small embossed pits in the header parts. Further, the efficiency of the recording track (format efficiency) is reduced because the header part has no grooves and cannot be used as a recording area. Therefore, the prior art is disadvantageous in realizing large capacity in the optical disk.
Another example of conventional methods for recording the address information by means of the wobbles of the groove portion without performing the recording at the sector address header part is described in the international standard ISO/IEC 16969.
In this example, the wobble groove that was frequency-modulated is used in order to record the address data. One round of the disk is composed of about 3,000 wobbles, and 7.5 periods of the wobbles are used to express one bit of the address data. To express bit “1,” 4 periods of the 7.5 periods are specified as the first half and the other 3.5 periods are specified as the second half. In other words, the first half is the wobbles of a high frequency and the second half is the wobbles of a low frequency. The frequency ratio is set at 8 to 7. Conversely, bit “0” is expressed by the 3.5-period low-frequency wobbles for the first half, and the 4-period high frequency wobbles for the second half. A set of 48 address bits forms an address codeword. 14 bits of the 48 bits of the address codeword are parities for error detection and the leading 4 bits are synchronization information used for establishing synchronization with this codeword. The breakdown of these four bits includes 30 periods of the wobbles (4.times.7.5), i.e., 12-period high-frequency wobbles, 3.5-period low-frequency wobbles, 4-period high-frequency wobbles, and 10.5-period low-frequency wobbles. The synchronization information can be identified from other data due to the fact that, in the synchronization information, wobbles of the same frequency longer than that of normal address bits by 4 periods or 3.5 periods whereas in a boundary of the normal address bits, wobbles of the same frequency only at 8-period (high-frequency) or 7-period (low-frequency) at most.
However, in the above-mentioned conventional technique, 1 address bit of the address data is expressed by 7.5-period wobbles and the difference in frequency between the first half part and the second halfpart is not large such that it is difficult to detect a boundary between the first half part and the second half part with high precision by increments of one unit of the wobble period based upon these wobbles. Further, since the synchronization information does not differ so much from other address bits, it is highly likely to be detected mistakenly. Moreover, the parity in the address codeword is 14 bits at most, which is sufficient only for checking errors but not for correcting errors such that the address information can not be reproduced if 1 bit in the address codeword is mistakenly detected It is necessary to secure sufficient S/N of the medium in order to ensure the reliability of address reproduction. When trying to apply this method to the high-density disk that needs the blue light source for reproduction, it is especially difficult to secure sufficient S/N because of the reduced efficiency of blue light detectors.
The first object of the present invention is to provide a high-performance optical disk such that the synchronization can be established easily for the address signal thereof so as to reproduce the address signal at a high speed.
The second object of the present invention is to provide an optical disk in which the address information that can be detected with high reliability.
The third object of the present invention is to provide a method for giving necessary medium information to the rewritable optical disk without using the embossed pits that are difficult to form.